Tripode constant velocity joint

ABSTRACT

A plunging, constant-velocity, tripode universal joint comprising a first rotary member ( 30 ) having three trunnions ( 31 ), a roller ( 34 ) having a cylindrical bore mounted on a spherical surface ( 32 ) on each trunnion so that the cylindrical bore engages the spherical surface and so that each roller can rotate, tilt and slide relative to its supporting trunnion, a second rotary member ( 40 ) having three grooves parallel to its rotary axis, each roller being engaged with one of the grooves, the engagement between each roller and track surfaces in its associated groove being at three points such that the orientation of the roller with respect to the second member is determined solely by said engagement and wherein when the joint is transmitting torque each roller ( 34 ) is only in contact with the track surface(s) ( 42, 43 ) through which the torque is being transmitted.

TECHNICAL FIELD

[0001] This invention relates to shudderless, tripode, plunging,constant-velocity, universal joints.

[0002] One type of such a constant-velocity, universal joint comprises afirst rotary member having a rotary axis, three trunnions extending fromthe first member, a roller mounted directly or indirectly on a sphericalsurface on each trunnion, such surface being provided by the trunnionitself or by a member rotatably mounted on the trunnion, the rollerincluding a cylindrical bore which engages the spherical surface so thateach roller can rotate, tilt and slide relative to its supportingtrunnion, the centres of the spherical surfaces on all the trunnionslying in a plane perpendicular to said rotary axis, a second rotarymember having a rotary axis, three grooves formed in said second rotarymember so as to extend parallel to the rotary axis of the second rotarymember, each roller being engaged with one of the grooves, theengagement between each roller and its associated groove being such thatthe orientation of the roller with respect to the second member isdetermined solely by said engagement. In such a joint there is relativeradial movement between the spherical surface of the trunnion and theroller.

BACKGROUND ART

[0003] One known form of shudderless, plunging, tripode joint is shownin cross section in FIG. 1. Referring to this FIG. there is a spider orinner member 10 of the joint which has three trunnions, one of which isshown at 11. The trunnion has a part-spherical surface 12 which receivesan inner roller 13 having a cylindrical bore 14. The inner roller 13 canslide and tilt relative to the trunnion and moves radially relative tothe centre of the trunnion when the joint is articulated and rotating.An outer roller 15 is mounted on the inner roller 13 to rotate relativethereto, there being a needle roller bearing 16 between the rollers 13and 15, The parts of the roller assembly are kept together by two rings17 and 18. The outer race of the joint is indicated at 19 and has threegrooves, each groove being formed by a pair of opposed tracks one ofwhich is shown at 20. The cross-sectional shape of each track is formedby two circular arcs which give a “Gothic arch” form and angularcontacts between the track and the roller. The centres 21 of thespherical surfaces of all of the trunnions lie in a plane perpendicularto the rotary axis 21 a of the spider.

[0004] When torque is to be transmitted from the outer race 19 to thespider 10 in an anticlockwise direction in FIG. 1, there is a reactionforce F0 which acts from the trunnion to the inner roller 13 and thenceto the outer roller 15. The force F0 is generally perpendicular to therotary axis of the roller 13, ignoring friction. There is two-pointcontact between the roller 15 and the right-hand track 20 and thereaction forces are shown at F1 and F2. The roller 15 is able to rotateabout the intersection of these forces at 22 and without furtherconstraint would be unstable. Because the roller 15 is free to tiltabout the intersection 22, in order for the roller to be stable it willalso engage the left-hand track so that there will be one or morereaction forces such as F3 or F4 on the circumference of the rollerand/or a force F5 on the upper surface of the roller which limits itstilting movement. These additional forces on the left-hand side of theroller are intermittent and are due to the fact that, as the trunnion 11moves up and down through the roller bore 14, the position of the roller15 relative to the outer race 19 can, in general, only be defined by twopoints of contact (i.e. those of the forces F1 and F2) instantaneouslywhen the trunnion is in a certain position with respect to the roller sothat other forces are generally required to determine the orientation ofthe roller. These intermittent other forces F3, F4 and F5 increase theresistance of the roller to rolling along the tracks and hence theplunge resistance of the joint, i.e the passive resistance. They mayalso cause the joint to generate a cyclic net axial force when itrotates with the rotary axes of the spider and outer race misaligned,this can give rise to shudder vibration in a vehicle in which the jointforms part of the driveline.

[0005] A similar arrangement is shown in FIG. 2 except that in this casethe trunnion 22 is cylindrical and an inner roller 23 provides thepart-spherical outer surface 24 which engages a cylindrical bore 25 ofthe outer roller 26. The inner roller 23 is mounted on the trunnion by aneedle roller bearing 27 and can not tilt or slide relative to thetrunnion. The outer roller 26 can rotate, tilt or slide relative to thetrunnion and to the part-spherical outer surface 24 which moves up anddown within the bore 25. The forces on the roller 26 are similar tothose described in relation to the joint shown in FIG. 1 and are shownby the same reference characters.

[0006] Because in each of the above examples the rollers 15 and 26 cantilt, slide and rotate relative to the trunnions and because the rollersare “shaped” to fit the grooves, the orientation of each roller withrespect to its associated groove is determined solely by the engagementof the roller with the groove. There are other configurations of tripodejoints in which the orientation of each roller relative to the outerrace is determined by the engagement of the roller with the groove.

[0007] There is described in WO-A-JP97/00017 a further type of tripodeshudderless joint and reference is made particularly to FIG. 17. In thisjoint, the tripode trunnions are cylindrical and mounted on eachtrunnion by needle roller bearings is an inner roller with apart-spherical outer surface. This engages an outer roller with an innerspherical surface. The outer roller can tilt with respect to the innerroller but any sliding radial movement takes place between the trunnionand the inner roller. The outer spherical surface does not move radiallywith respect to the outer roller so that, unlike the joint of thecurrent invention as will be described below, the force between thetrunnion and the roller assembly acts at a fixed position relative tothe roller assembly. Therefore there may be no tendency for the rollerassembly to twist about an axis parallel to the axis of the secondmember.

[0008] In the prior joint the outer roller has a trapezoidal outersurface which engages a track surface of corresponding shape. It issuggested that contact may take place between three faces of the outerroller and the track but this would seem to require a very accurate fitbetween roller and track. This is acknowledged by the fact that theroller and track inclined surfaces are described as facing one anotherwith “a gapless contact or a very small gap”. In practice the twotrapezoids will generally only be in contact on one or two of theirsides.

[0009] WO-A-JP97/00017 also describes how the rollers are only incontact with the track surface through which torque is beingtransmitted. This may be achieved by making the driving track (i.e. thetrack through which torque is transmitted) narrower than the othertrack. This results in asymmetric grooves and a requirement fordifferent components to be used on the left and right hand sides of avehicle.

DISCLOSURE OF THE INVENTION

[0010] An object of the present invention is to reduce the resistance ofthe rollers to rolling along the tracks and thus to reduce the plungeresistance whilst ensuring that the rollers are stable and that there iscontinuous three-point contact between the roller and the track, withoutrequiring the profiles of the roller and the track to be matched withextreme accuracy.

[0011] Another object of the invention is to provide a shudderless,tripode joint in which the stability of each roller is determined solelyby its engagement with the driving track and there are no intermittentcontacts between the roller and the other track.

[0012] Another object of the invention is to reduce the NVH (noise,vibration, harshness) associated with clearance in the joint by dampingthe backlash movement of the roller when the torque is reversed.

[0013] Another object of the invention is to provide a joint in whichthe grooves to receive the rollers have a simple form making the secondrotary member or outer race easy to manufacture.

[0014] Another object of the invention is to provide a joint in whichthe grooves are symmetrical.

[0015] Another object of the invention is to provide a joint in whichall the contacts between the roller and the track are Hertzian (ashereinafter described);

[0016] According to one aspect of the invention we provide a plunging,constant-velocity universal joint comprising a first rotary memberhaving a rotary axis, three trunnions extending from the first member, aroller mounted directly or indirectly on a spherical surface on eachtrunnion, such surface being provided by the trunnion itself or by amember rotatably mounted on the trunnion, the roller including acylindrical bore which engages the spherical surface so that each rollercan rotate, tilt and slide relative to its supporting trunnion, thecentres of the spherical surfaces on all the trunnions lying in a planeperpendicular to said rotary axis, a second rotary member having arotary axis, three grooves formed in said second rotary member so as toextend parallel to the rotary axis of the second rotary member, eachgroove comprising spaced-apart track surfaces which extend parallel tothe rotary axis of the second member, each roller being engaged with atrack surface in one of the grooves, the engagement between each rollerand its associated track surface through which torque is beingtransmitted being at three points which fully determine the roller'sorientation with respect to the second member and wherein, when thejoint is transmitting torque, each roller is only in contact with thetrack surface through which the torque is being transmitted.

[0017] According to another aspect of the invention we provide aplunging, constant-velocity universal joint comprising a first rotarymember having a rotary axis, three trunnions extending from the firstmember, a roller mounted directly or indirectly on a spherical surfaceon each trunnion, such surface being provided by the trunnion itself orby a member rotatably mounted on the trunnion, the roller including acylindrical bore which engages the spherical surface so that each rollercan rotate, tilt and slide relative to its supporting trunnion, thecentres of the spherical surfaces on all the trunnions lying in a planeperpendicular to said rotary axis, a second rotary member having arotary axis, three grooves formed in said second rotary member so as toextend parallel to the rotary axis of the second rotary member, eachgroove comprising spaced-apart track surfaces which extend parallel tothe rotary axis of the, second member, each roller being engaged with atrack surface in one of the grooves, the engagement between each rollerand its associated track surface through which torque is beingtransmitted being at three points which fully determine the roller'sorientation with respect to the second member, wherein the contactvectors of the reaction forces at said three points, when projected onto a common plane perpendicular to the rotary axis of the second member,form a triangle, wherein the contact vector of the force between theroller and the spherical surface, when projected onto said common plane,intersects the two sides of the triangle formed by the projected contactvectors of the reaction forces acting at the radially innermost andradially outermost of said points, the radial positions of said pointsbeing measured with respect to the rotary axis of the second member andwherein, when the joint is transmitting torque, each roller is only incontact with the track surface through which the torque is beingtransmitted.

[0018] Preferably the track surfaces in each groove are symmetrical withrespect to a plane (the plane of symmetry) containing the rotary axis ofthe second member.

[0019] Preferably two of said contact vectors of the reaction forceswhen projected on to said common plane intersect on the plane ofsymmetry, said two contact vectors being one of the reaction forcesacting at the radially innermost or radially outermost of said pointsand the contact vector of the reaction force acting at the radiallyintermediate point.

[0020] A first track surface on which the radially innermost point orthe radially outermost point and the radially intermediate point issituated may be cylindrical. Said first track surfaces on each side ofthe groove may be parts of the same cylinder.

[0021] The radially innermost or radially outermost point which is noton the first track surface may be on a second track surface which iscylindrical. The first and second track surfaces may have a commontangent where they meet.

[0022] When the joint is transmitting torque the rotary axis of eachroller may be tilted with respect to the plane of symmetry of itsassociated tracks. Thus when the direction of torque transfer throughthe joint reverses, each roller moves into contact with a track surfacethrough which torque is then being transferred and tilts about an axisparallel to the rotary axis of the second member until its orientationis determined by said three-point contact. The tilt movement of eachroller is preferably in a sense opposite to the direction of rotation ofthe first member after the direction of torque transfer has beenreversed.

[0023] Each trunnion may have a part spherical surface engaged with acylindrical bore of an inner rotary member on which the roller, isrotatably mounted. Alternatively each trunnion may have a cylindricalsurface on which is rotatably mounted an inner rotary member having aspherical outer surface engaged with a cylindrical bore of the roller.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The invention will now be described in detail by way of examplewith reference to the accompanying drawings in which:

[0025]FIG. 3 is a cross-section through part of a joint constituting oneembodiment of the invention:

[0026]FIG. 4 is a diagram showing what happens when the roller skews;

[0027]FIGS. 5a and 5 b are diagrams showing the orientations of thethree rollers when torque is being transmitted from the outer member tothe inner member in clockwise and anticlockwise directions respectively;

[0028]FIG. 6 is a cross-section similar to FIG. 3 of a second embodimentof the invention; and

[0029]FIG. 7 is a cross section through a complete joint embodying theinvention; and

[0030]FIGS. 8, 9 and 10 are views similar to FIG. 3 of three furtherembodiments.

BEST MODE FOR CARRYING OUT THE INVENTION

[0031] Referring now to FIG. 3, the inner member or spider of the jointis indicated at 30 and one of the trunnions at 31. The trunnion has apart-spherical external surface 32, the centre of which is shown at 32a. There are three trunnions as shown in FIG. 7 and tile centres of thespherical surfaces of all the trunnions lie in a plane perpendicular tothe rotary axis 30 a of the inner member. Mounted on each trunnion is aninner roller 33 which has a cylindrical bore to engage thepart-spherical surface 32. An outer roller is indicated at 34 and ismounted on the inner roller by a needle roller bearing 35 and retainedin position by rings 36.

[0032] The outer race 40 of the joint has three grooves as shown in FIG.7, each groove providing a pair of tracks, one pair of which is shown inFIG. 3, the track surfaces being indicated at 41 and 42. These surfacesform parts of the same cylinder but may be of other shapes. The tracks41 and 42 are symmetrically arranged with respect to a plane (the planeof symmetry) containing the rotary axis of the outer race and the line45. The root 43 of the track is also a cylindrical surface having itscentre at 43 a. The tracks in the other grooves are similarly arranged.

[0033] As shown in FIG. 3 the outer roller 34 has an externalcircumferential surface 37 which engages the track 42 and a surface 39which engages the root 43 so that there is three-point contact betweenthe roller and the track surface 42 and the root 43. Thus assuming thattorque is being transmitted from the trunnion 31 to the outer race 41 ina clockwise sense there is a force F0 between the spherical surface 32and the inner roller 33 and two reaction forces F2 and F3 at the twopoints where the outer roller 34 engages the track 42. There is also aforce F1 where the surface 39 at the top of the roller engages part ofthe track root 43. The roller is thus determined in its orientation bythis three-point contact represented by the arrows F1, F2 and F3. Thecontact vectors, i.e. the lines of action, of the forces F2 and F3 ifprojected on to a common plane perpendicular to the rotary axis 30 aintersect at O, the centre of curvature of the tracks 41 and 42. In theabsence of the force F1 the roller could rotate about the point O andwould be unstable. The force F1, if projected on to said common plane,intersects the projected contact vectors of the forces F2 and F3 at Eand D respectively. The points O, D and E form a triangle and thecontact vector, line of action, of the force F0 (which acts through thecentre 32 a of the spherical surface 59) intersects two sides of thetriangle, i.e. OD and DE. This arrangement, described in more detailbelow ensures that the roller is stable and is maintained in positionwith its three point contact. It is to be noted that of the trianglesides intersected by the contact vector of the force F0 the side OD isthe contact vector of the radially innermost force F3 and the line DE isthe contact vector of the radially outermost force F1, where the radialpositions are measured with respect to the rotary axis 30 a of thesecond member. As long as the force F0, when projected on to said commonplane, intersects side DE of the triangle ODE, contact is maintained atpoints associated with forces F2 and F3. In addition, as long as theforce F0 also intersects side OD of the triangle ODE, the moment on theroller about point O ensures contact at the point associated with forceF1. In other words, these conditions ensure that the values of forcesF1, F2 and F3 are positive so that contact is maintained at these threepoints.

[0034] Each of the forces transmitted to the outer race 40 from atrunnion 31 and reacted by the forces F1, F2 and F3 have a tangentialcomponent acting in the positive torque-transmitting direction. Thepoint forces are preferably Hertzian, i.e. there are at least twoprincipal radii of curvature of the outer surface 37 and the tracksurface 42 which results in there being discrete elliptical contactareas.

[0035] It will be seen that there is a clearance 44 (much exaggerated inthe drawing) between the left-hand track 41 and the surface 37 of theroller. The roller is therefore only in contact with the track throughwhich torque is being transferred, i.e. in the drawing, the track whichis to the right of the plane of symmetry of the tracks 41, 42. The wholeof the circumference of the roller 34, therefore to the left of suchplane is out of contact with the track 41 and there will be nointermittent contacts therewith as in FIGS. 1 and 2. The rotary axis ofthe roller is not parallel to the line 45 which passes through the point43 a the centre of the root cylindrical surface 43, and the sphericalcentre O. During operation of the joint the point of contact between thespherical surface 32 of the trunnion and the cylindrical bore of theroller 33 will move radially.

[0036] If the direction of torque transfer now reverses, the surface 37of the roller 34 will come into engagement with the left-hand track 41and the roller will then tilt so that the left-hand portion of thesurface 39 comes into contact with the root 43 of the track. The rollerwill move out of contact with the track 42. This tilting of the rolleris in a direction opposite to the direction of rotation of the innermember 30 of the joint once the reversal of torque transmission hastaken place. It is thought that this motion and the sliding of theroller across the track 41, following the initial impact, will tend toabsorb the energy of the impact and hence reduce the effects of backlashwhich can give rise to noise and vibration problems when the joint isinstalled in a vehicle.

[0037] If the roller skews, i.e. rotates about an axis perpendicular tothe roller axis and to the rotary) axis of the outer member then, asshown in FIG. 4, the contact point between the surface 39 on the end ofthe roller and the track root 43 is free to move from the position 46 tothe position 47 or the position 48 or somewhere between these twopositions. Movement of this contact point will limit the skew motion ofthe roller.

[0038]FIG. 5a shows the three rollers in their tilted orientations astorque is being transmitted from the outer member of the joint to theinner member in a clockwise sense and FIG. 5b shows the rollerorientations when torque is being transmitted in an anticlockwise sense.When the torque is reversed the rollers tilt in the direction oppositeto the direction of rotation of the inner member 30 once reversal oftorque has taken place and engage the tracks on the other sides of thegrooves so that there is a clearance between those faces of the rollerwhich are not transmitting torque and the tracks.

[0039]FIG. 6 shows a joint similar to FIG. 2 but which embodies theinvention. Thus referring to FIG. 6 the inner member of the joint isindicated at 50 and a trunnion at 51. The trunnion is cylindrical andcarries a needle roller bearing 52 which in turn carries an inner roller53 having an external spherical surface 54. An outer roller 55 has acylindrical bore 56 which engages the spherical surface 54.

[0040] The roller 55 is of the same shape as that described in relationto FIG. 3 as is the track 57 in the outer member 58 of the joint. Againthere is three-point Hertzian contact indicated by the arrows F1, F2 andF3 and the operation of this joint is as described in relation to thejoint of FIG. 3.

[0041] As in FIG. 3 the contact vectors of the forces F2 and F3, whenprojected on to a common plane perpendicular to the rotary axis of thesecond member intersect at O, the centre of curvature of the tracks 57.The contact vector of the force F1 when projected on to said commonplane intersects the force vectors of the forces F2 and F3 at E and Dand the points O, E and D are apices of a triangle. As shown the contactvector of the force F0 intersects the triangle sides OD and DE so thatthe roller 55 is in stable equilibrium under the influence of the forcesF1, F2 and F3. As described in relation to FIG. 3 the sides OD and DEwhich are intersected by the contact vector of the force F0 (which actsthrough the centre S of the spherical surface 54) are the contactvectors of the radially innermost force, F3, and the radially outermostforce, F1. As in FIG. 3 also the roller is out of contact with theleft-hand track 57.

[0042] Referring now to FIG. 8, this shows a joint similar to that shownin FIG. 3 except that in this case the roller is not twisted. Thus theinner member of the joint is indicated at 60 and a trunnion at 61. Thereare three such trunnions equi-angularly spaced around the rotary axis 62of the inner member. The trunnion has an outer spherical surface 63which engages the cylindrical bore 64 of an inner roller 65. The innerroller is surrounded by a ring of needle rollers 66 on which runs theouter roller 67, the whole being held together by circlips 68 asdescribed in relation to FIG. 3.

[0043] The outer member is indicated at 69 and has three grooves asbefore one of which is shown at 70. Each groove has two tracks 71 and 72which are formed of cylindrical surfaces and form part of the samecylinder the centre of which is at O. There is a second cylindricaltrack surface on each side of the groove shown at 74 and 75respectively. The surfaces 71 and 74 are co-tangential as are thesurfaces 72 and 75.

[0044] As in FIG. 3, the roller 67 is in contact with the track surfaces71 and 74 but is out of contact with the track surfaces 72 and 75. Thisassumes that torque is being transferred in a anticlockwise directionfrom the outer member to the inner member.

[0045] The contact vector of the trunnion to roller force is shown atF0. There are three reaction forces FA, FB and FC which act at threepoints A, B and C respectively Point A is on the cylindrical surface 74and points B and C are on the cylindrical surface 71.

[0046] The contact vectors of the forces FB and FC intersect at O, i.e.the axis of the cylindrical surfaces 71 and 72. If these force vectorsare projected onto a common plane and the force vector of the force FAwhich acts at a point A on the cylindrical surface 74 is also projectedonto the same common plane, one gets a triangle whose apices are O, Eand D. It will be seen that the contact vector of the force F0 (whichacts through the centre S of the spherical surface 63) intersects thelines OD and DE. This is a necessary condition for the roller 67 toremain in three-point contact with the surfaces 71 and 74 at the pointsA, B and C. It will be noted as described above that the sides which areintersected by the contact vector of the force F0 are the sides formedby the contact vector of the radially innermost force, FC and of theradially outermost force, FA. The contact vector of the force F0 doesnot intersect the contact vector of the intermediate force FB.

[0047] If the direction of torque transfer through the joint reversesthen the roller 67 will come into contact with the surfaces 72 and 75and out of contact with the surfaces 71 and 74. It is to be noted that,ignoring friction, the contact vector force F0 acts through the centreof the spherical surface 63.

[0048] Like parts in FIGS. 8 and 9 are shown by the same referencenumbers. FIG. 9 shows a similar arrangement to FIG. 8 except that inthis case the intersection point O of the contact vectors of the forcesFB and FC does not lie on the plane of symmetry of the tracks. Asbefore, the contact vector of the force F0 intersects the sides OD andDE which are the sides of the triangle formed by the contact vectors ofthe radially innermost and outermost forces, i.e. FC and FArespectively, when the forces are projected on to a common planeperpendicular to the rotary axis of the outer member which is shown bythe point 62 which is the same as the rotary axis of the inner member.It will be seen that in each of FIGS. 8 and 9 the intersection O of theforces FB and FC is at a point radially outwardly of the centre of thespherical surface of the trunnion.

[0049]FIG. 10 shows a further arrangement in which the intersection O ofthe forces FC and FB is at a position which is radially inwardly of thecentre of the trunnion. In this embodiment, the point A, where the forceFA acts, the force FA being that with the greatest roller-axialcomponent, is located radially inwardly of the points B and C where theforces FB and FA act. In FIG. 10 like parts are indicated by the samereference numerals as in FIG. 8. In FIG. 10, ignoring friction, thecontact vector of the force F0 acts through the centres of the sphericalsurface 63.

[0050] The contact vectors of the forces FB and FC act through thepoints B and C respectively and when projected on to a common planeperpendicular to the rotary axis of the outer member intersect at thepoint O which is radially inward from the centres of the trunnionsphere. In this case the contact vector of the force F0 intersects thesides OE and ED of the triangle. These sides are those provided by theradially outermost force FC and the radially innermost force FA. Asbefore the contact force vector of the force F0 does not intersect thecontact vector of the force FB.

[0051] Preferably at all the contact points A, B and C the contacts areHertzian since the contact surfaces have at least two identifiable radiiof curvature. In fact the contact at the points A, B and C have threeradii of curvature, two associated with the roller 67 and one associatedwith the tracks 71, 74 and 72, 75. Such contacts should promote theingress of lubricant into the contact zones and the resulting formationof a lubricant film should reduce the rolling resistance of the roller.

[0052] The invention thus provides a joint which fulfills theabove-mentioned objects, has a low plunge resistance and low NVH.

1. A plunging, constant-velocity universal joint comprising a firstrotary member having a rotary axis, three trunnions extending from thefirst member, a roller mounted directly or indirectly on a sphericalsurface on each trunnion, such surface being provided by the trunnionitself or by a member rotatably mounted on the trunnion, the rollerincluding a cylindrical bore which engages the spherical surface so thateach roller can rotate, tilt and slide relative to its supportingtrunnion, the centres of the spherical surfaces on all the trunnionslying in a plane perpendicular to said rotary axis, a second rotarymember having a rotary axis, three grooves formed in said second rotarymember so as to extend parallel to the rotary axis of the second rotarymember, each groove comprising spaced-apart track surfaces which extendparallel to the rotary axis of the second member, each roller beingengaged with a track surface in one of the grooves, the engagementbetween each roller and its associated track surface through whichtorque is being transrmitted being at three points which fully determinethe roller's orientation with respect to the second member, and wherein,when the joint is transmitting torque, each roller is only in contactwith the track surface through which the torque is being transmitted. 2.A plunging, constant-velocity universal joint comprising a first rotarymember having a rotary axis, three trunnions extending from the firstmember, a roller mounted directly or indirectly on a spherical surfaceon each trunnion, such surface being provided by the trunnion itself orby a member rotatably mounted on the trunnion, the roller including acylindrical bore which engages the spherical surface so that each rollercan rotate, tilt and slide relative to its supporting trunnion, thecentres of the spherical surfaces on all the trunnions lying in a planeperpendicular to said rotary axis, a second rotary member having arotary axis, three grooves formed in said second rotary member so as toextend parallel to the rotary axis of the second rotary member, eachgroove comprising spaced-apart track surfaces which extend parallel tothe rotary axis of the second member, each roller being engaged with atrack surface in one of the grooves, the engagement between each rollerand its associated track surface through which torque is beingtransmitted being at three points which fully determine the roller'sorientation with respect to the second member, wherein the contactvectors of the reaction forces at said three points, when projected onto a common plane perpendicular to the rotary axis of the second member,form a triangle, wherein the contact vector of the force between theroller and the spherical surface, when projected onto said common plane,intersects the two sides of the triangle formed by the projected contactvectors of the reaction forces acting at the radially innermost andradially outermost of said points, the radial positions of said pointsbeing measured with respect to the rotary axis of the second member, andwherein, when the joint is transmitting torque, each roller is only incontact with the track surface through, which the torque is beingtransmitted.
 3. A joint as claimed in claim 1 or claim 2 wherein thetrack surfaces in each groove are symmetrical with respect to a plane(the plane of symmetry) containing the rotary axis of the second member.4. A joint as claimed in claim 2 wherein two of said contact vectors ofthe reaction forces when projected on to said common plane intersect onthe plane of symmetry, said two contact vectors being one of thereaction forces acting at the radially innermost or radially outermostof said points and the contact vector of the reaction force acting atthe radially intermediate point.
 5. A joint according to any precedingclaim wherein a first track surface on which the radially innermostpoint or the radially outermost point and the radially intermediatepoint is situated is cylindrical.
 6. A joint according to claim 5wherein said first track surfaces on each side of a groove are parts ofthe same cylinder.
 7. A joint according to claim 5 or claim 6 whereinthe radially innermost or radially outermost point which is not on thefirst track surface is on a second track surface which is cylindrical.8. A joint according to claim 7 wherein the first and second tracksurfaces have a common tangent where they meet.
 9. A joint as claimed inclaim 2 or in any of claims 3 to 8 when dependent from claim 2, whereinwhen the joint is transmitting torque the rotary axis of each roller istilted with respect to the plane of symmetry of its associated tracks.10. A joint as claimed in claim 9 wherein, when the direction of torquetransfer through the joint reverses, each roller moves into contact witha track surface through which torque is then being transferred and tiltsabout an axis parallel to the rotary axis of the second member until itsorientation is determined by said three-point contact.
 11. A joint asclaimed in claim 10 wherein the tilt movement of each roller is in asense opposite to the direction of rotation of the first member afterthe direction of torque transfer has been reversed.
 12. A joint asclaimed in any preceding claim wherein each trunnion has a partspherical surface engaged with a cylindrical bore of an inner rotarymember on which the roller is rotatably mounted.
 13. A joint as claimedin claims 1 to 11 wherein each trunnion has a cylindrical surface onwhich is rotatably mounted an inner rotary member having a sphericalouter surface engaged with a cylindrical bore of the roller.
 14. A jointas claimed in any preceding claim wherein all the contacts between therollers and the tracks are Hertzian, i.e. the roller and the trackshave, between them, at least two identifiable radii of curvature wherethey touch.
 15. A joint substantially as hereinbefore described withreference to and as shown in any of FIGS. 3, 6, 8, 9 or 10 of theaccompanying drawings.